The invention relates to a method for determining the optical quality of a transparent plate, particularly a float glass plate, in which two parallel light beams having a reciprocal spacing are directed onto the plate under an acute angle, relative to the plate normal, the light beams reflected by the plate are received separately by means of a detector means having a photosensitive detector and the direction of the reflected light beams is evaluated. The invention also relates to an apparatus for determining the optical quality of a transparent plate, with a laser light source and a beam splitting means for producing two parallel light beams having a reciprocal spacing, which are directed onto the plate under an acute angle, relative to the plate normal, a detector means having a photosensitive detector for the separate determination of the reflected light beams and with an evaluating means for determining the optical quality of the plate from the signals of the detector means.
In the manufacture of transparent plates and in particular in flat glass manufacture, it is necessary to regularly check the optical quality of the material produced and this should take place as soon as possible after production so that, if necessary, in good time production parameters can be modified or faulty plates eliminated.
Hereinafter both the invention and the prior art are explained relative to flat glass testing. However, it can also be used for testing the optical quality of other transparent, plate-like products, e.g. made from plastic.
Apart from the detection of glass defects, such as e.g. homogeneity fluctuations, glass bubbles or inclusions, particular interest is attached to the planeity of glass surfaces. Problems with respect to the planeity are in particular observed at right angles to the drawing direction of a glass ribbon in the form of a so-called draw line. The consequence of such planeity divergences are optical distortions, which lead to a deflection of light beams in reflection and transmission.
A quantitative measurement quantity for the optical quality of glass plates is e.g. the optical power thereof, namely in the form of the optical power in reflection of the individual plate surfaces and the optical power in transmission of the plate. The optical power is defined as the derivation of the angle observed, i.e. the reflection angle or the transmission angle after the location. The greater the local curvature of the glass surface the larger the optical power.
According to the prior art there has been a subjective testing of the optical quality of glass plates in the form of an inspection by especially trained personnel. Light is irradiated through the glass plate to be tested onto a projection screen. Planeity divergences lead to local brightness fluctuations (lens effect) and can in this way be quantitatively evaluated. However, there are also methods and apparatuses for the quantitative evaluation of the optical quality of glass plates.
German patent 23 61 209, on which the invention is based, describes a method for determining the optical quality of glass plates, in which a laser light beam is split by means of a beam splitter into two parallel light beams. The two light beams strike the glass plate under a predetermined acute angle, relative to the plate normal and are reflected either on the front surface thereof and subsequently on a concave mirror or on a concave mirror located behind the glass plate in the direction of a photosensitive detector, which is installed within a rotating, drum-shaped slit diaphragm. What is evaluated is the time interval between the two light pulses recorded by the photosensitive detector, said time interval being correlated with the reflection angles of the two reflected beams. The slit diaphragm as a component of the detector means ensures that the two reflected beams are separately, i.e. in time succession, detected by a non-position-sensitive photosensitive detector of the detector means.
However, the known method suffers from disadvantages. During a measuring process it only makes it possible to test one of the quantities which are of interest, i.e. either only the optical quality of one of the two glass surfaces or only the distortion in transmission.
For evaluating the optical quality of both surfaces of a glass plate and the deflection in transmission it is necessary to perform three successive, different measurements between which the glass plate must be inverted and cleaned. As a result a large amount of time is required for a complete measurement of a plate. In addition, it is still only possible to carry out a qualitative or semiqualitative evaluation of the measurement diagram, which must be performed by an experienced person. It is not possible with the known apparatus to determine standardized measurement quantities, such as e.g. the optical power.
In addition, the known apparatus has a considerable size with all the resulting disadvantages. It also requires the arrangement of parts of the measuring apparatus on both sides of the glass. As the measuring accuracy is to a large extent dependent on the constancy of the rotating speed of the slit diaphragm, the known apparatus is relatively fault-prone.